Shaheen on Saturday sent a letter to Brown asking him to sign the "People's Pledge," an identical document to the one he agreed to while running against Sen. Elizabeth Warren (D-Mass.) in 2012.

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"I have signed and attached two copies of an agreement with the exact same terms for the New Hampshire 2014 Senate race," Shaheen said in a letter to Brown. "I hope you will join me in once again committing to the same People’s Pledge you signed in Massachusetts and limiting the influence of outside groups in New Hampshire this year."

The move comes a day after Brown announced he was opening an exploratory committee for the New Hampshire Senate race and began a listening tour in the state to gauge support.

Brown is seen as one of the only Republicans who can make New Hampshire competitive in the 2014 midterms, due to his star power and heavy fundraising capabilities.

The pledge aims to take away the incentive for independent expenditure groups, individuals and super-PACs from spending in the race to either oppose or support a candidate.

If outside ads are run in the state, the pledge would require the candidate that benefits from the ad to pay 50 percent of the cost of the ad buy to charity. The pledge would relate to broadcast and online advertising and would cover negative and positive outside ads.

It does not, however, address outside spending on direct mail or field work in the state.

"If you would please sign one of the enclosed copies and return it to me at Shaheen for Senate as soon as possible, the people of New Hampshire will have the assurance that their voices will not be drowned out by third-party expenditures," Shaheen said.

According to a Sheheen aide, the conservative group Americans for Prosperity has already reserved or spent $1.2 million against the senator.

In a statement, Brown blasted Shaheen's offer as "self-serving and hypocritical." While he did not specifically rule out signing the pledge, he made it clear he saw it as a stunt.

"Before I even thought of becoming a candidate, Jeanne Shaheen's allies in Washington were running negative ads against me for months. And right now, while I'm meeting with the people of New Hampshire, she is on the West Coast raising money so third-party groups in DC will have money to run even more outside negative ads against me," he said. "It's hard to view Jeanne Shaheen's actions as anything other than hypocritical and self-serving. The people of New Hampshire can see through the Washington-style game she is playing.”

Brown signed the pledge in 2012 in one of the most expensive races in the country. After the pledge was signed with Warren, outside spending was only about one tenth of what the two campaigns spent, according to the Center for Responsive Politics. Outside spending totaled about $8 million compared to $77 million spent by Brown and Warren.